In recent years, laser light sources are receiving attention as one light source of a projection-type image display apparatus of which projectors are representative. A laser light source has several advantages. First, laser light emitted from a laser light source features superior directivity and therefore features high optical utility efficiency. In addition, laser light is monochromatic and therefore can broaden the color reproduction region. A laser light source also features low power consumption and long life.
FIG. 1 shows a schematic block diagram of a projector that uses a laser light source. Projector 1 shown in FIG. 1 includes at least: laser light sources 2a-2c corresponding to each of the primary color signals R/G/B; collimator lenses 3a-3c, light tunnels 4a-4c, optical modulation elements (liquid crystal panels) 5a-5c, dichroic prism 6, and projection lens 7.
Polarization beam splitters (PBS) 8a-8c are arranged between each of laser light sources 2a-2c and each of collimator lenses 3a-3c. In addition, incidence-side polarizing plates 9a-9c are arranged on the light incidence side of each of liquid crystal panels 5a-5c. Emission-side polarizing plates 10a-10c are arranged on the light emission sides of each of liquid crystal panels 5a-5c. 
The operation of projector 1 shown in FIG. 1 is next summarized. Laser light beams 12a-12c emitted from each of laser light sources 2a-2c, respectively, are converted to specific linearly polarized light by polarization beam splitters (PBS) 8a-8c and pass through collimator lenses 3a-3c, respectively. Laser light beams 12a-12c that have passed through collimator lenses 3a-3c are directed into light tunnels 4a-4c. The beam diameters of laser light beams 12a-12c that have passed through collimator lenses 3a-3c are gradually enlarged until being irradiated into light tunnels 4a-4c. 
Light tunnels 4a-4c are hollow prisms. A reflective film is applied by vapor deposition to the inner wall surfaces of light tunnels 4a-4c. Laser light beams 12a-12c irradiated into light tunnels 4a-4c from one opening of each of light tunnels 4a-4c, respectively, advance toward the other opening while being repeatedly reflected inside light tunnels 4a-4c. In the process of advancing inside light tunnels 4a-4c, not only is the luminance distribution in the luminous flux cross-sections of laser light beams 12a-12c equalized, but the sectional profiles are reshaped into a rectangular form.
Laser light beams 12a-12c that are emitted from each of light tunnels 4a-4c are irradiated into corresponding liquid crystal panels 5a-5c, respectively. Laser light beams 12a-12c that have been irradiated into liquid crystal panels 5a-5c undergo optical modulation according to image signals. The light that has undergone optical modulation is synthesized by dichroic prism 6 and enlarged and projected onto screen 11 by way of projection lens 7.
However, when coherent light such as laser light is irradiated onto a rough surface (such as a screen) having unevenness that is greater than the wavelength of the light, a mottled light pattern referred to as a “speckle pattern” or “speckle” is produced. More specifically, light of a single wavelength that is scattered at each point on a rough surface overlaps irregularly at each point on the observed surface to produce a complicated interference pattern.
Thus, when an image is projected onto a screen by a projector that uses a laser light source, the laser light is diffused on the screen surface and strong random noise (speckled noise) is produced. When this speckle is formed as an image on an observer's retina in this case, the speckle is perceived as unfocused mottled flickering, and this causes discomfort and fatigue for the observer. The observer further senses extreme degradation of the image quality.
In the field of projectors that employ laser light sources, various methods have been proposed for reducing the above-described speckle noise.
Typically, two approaches exist as methods for reducing speckle noise. One involves making the laser light incoherent (Approach 1), and other involves reducing the perceived speckle (Approach 2).
Approach 1 is a method of canceling the coherence of laser light to convert to incoherent light. The broadening of wavelength width by means of high-frequency superimposition, the multiplexing of laser light having a delay that is greater than the coherence length, or the overlapping of orthogonal polarized light all pertain to Approach 1. Essentially, Approach 1 is a method of altering the characteristics of light to control the generation of speckle.
In contrast, Approach 2 is a method for reducing apparent speckle by repeatedly superimposing the (integral) speckle pattern in an image at time intervals (<40 msec) that are indistinguishable to the human eye to equalize speckle noise. Methods of vibrating the screen or optics components belong to Approach 2. Methods that belong to Approach 2 do not alter the characteristics of light, and speckle is therefore generated. Approach 2 is a method that takes advantage of an illusion in the human brain to make speckle imperceptible to the eye.
Of the methods that pertain to Approach 2 (reducing perceptible speckle), the method of reducing speckle noise by vibrating optics components is taken up in the present specification.
FIG. 2 is a perspective view showing a first technique for reducing speckle noise. FIG. 2(a) shows an example of the first technique, and FIG. 2(b) shows another example. The details of the first technique are disclosed in JP-A-H11-064789. In the example shown in FIG. 2(a), optical integrator 17a composed of two fly-eye lenses 13c and 13d rotates around an optical axis. When the optics rotate, the speckle pattern moves temporally and spatially in the optics, the speckle that is image-formed on the retina is integrated, and the apparent speckle noise is reduced. In the example shown in FIG. 2(b), on the other hand, a similar effect is obtained by the rotation of rod-type optical integrator 19a (a transparent medium such as glass having a rectangular cross-section) around the optical axis.
FIG. 3 is a block diagram showing a second technique for reducing speckle noise. FIG. 3(a) shows an example of the second technique, and FIG. 3(b) shows another example. The details of the second technique are disclosed in JP-A-H07-297111. In the example shown in FIG. 3(a), diffusion plate 16b that is caused to rotate by motor 20 is arranged midway on an optical path. When diffuser 16b rotates, the scattering state on the optical path changes and the speckle pattern vibrates temporally and spatially, whereby the speckle that is formed as an image on the retina is integrated and the apparent speckle noise is reduced. In the example shown in FIG. 3(b), diffuser 16c that is arranged midway on an optical path is caused to vibrate by transducer 23. When diffusion plate 16c vibrates, the apparent speckle noise is reduced due to the same principles as previously described.
FIG. 4 is a sectional view showing a third technique for reducing speckle noise. FIG. 4(a) shows an example of the third technique, and FIG. 4(b) shows another example. The details of the third technique are disclosed in JP-A-2003-098476. In the example shown in FIG. 4(a), diffuser 16d is arranged between beam expanding optics 25 that include expanding lens (collimator lens 3f) and collimator lens 3g and beam shaping optics 27 that include two fly-eye lenses 13e and 13f and condenser lenses 14h and 14i. Diffuser 16d is caused to vibrate by movement-inducing means 26a. When diffuser 16d vibrates, the speckle pattern vibrates temporally and spatially, whereby the speckle that is formed as an image on the retina is integrated and the apparent speckle noise is reduced. In addition, in the example shown in FIG. 4(b), diffuser 16e is also arranged between beam shaping optics 27 and spatial optical modulation element 5f. Diffusers 16d and 16e are caused to vibrate by movement-inducing means 26a and 26b. 
FIG. 5 is a structural diagram showing a fourth technique for reducing speckle noise. FIG. 5(a) shows one example of the fourth technique and FIG. 5(b) shows another example. The details of the fourth technique are disclosed in WO2005/008330. In the example shown in FIG. 5(a), diffuser 16f arranged midway in an optical path is connected to diffuser vibration section 28a. Diffuser vibration section 28a causes diffuser 16f to vibrate at a vibration speed V. Vibration speed V is set to satisfy the relation V>d×30 where d is the particle size of diffuser 16f. WO2005/008330 discloses control of the diffusion angle of a diffuser based on the relation between the numerical aperture of the illumination optics and the F-number of the projection lens to suppress the optical loss of laser light caused by a diffuser. In the example shown in FIG. 5(b), rod-type optical integrators 19b are used in place of two fly-eye lenses 13g and 13h. 